1. Technical Field
The invention relates to data transmission via optical signals in general and especially to a method and a device for automatic control of a delay line.
2. Description of the Related Art
For data transmissions in telecommunication and for data transfer optical signals are increasingly employed. In the past years the speed of data transmission via fiber optical connections increased quite considerably.
To achieve high bandwidths in optical data transmission advanced modulation formats are required. While amplitude modulation is still predominant, advanced modulation formats often make use of an additional phase modulation. Among the advanced modulation formats which will be implemented first due to their simplicity are duobinary coding and differential phase shift keying (DPSK). For duobinary coding and DPSK, as for some other modulation formats, a logical EXOR or “modulo 2” operation of the signal can be necessary in the modulator or demodulator.
The realization of such operation by means of an electrical circuit faces difficulties, but in the optical domain it can be realized by using a delay line interferometer (DLI).
Optical DLIs require high accuracy with respect to delay-to-bitrate mismatch and delay line phase offset. While delay-to-bitrate mismatch does not affect the interference properties of the DLI and also leads to only minor signal distortion, the signal is sensitive to delay line phase offset. This results from the fact that the functionality of an interferometer fundamentally relies on the exactly constructive and destructive interference between two optical fields.
As delay line phase offset or delay line phase tuning depends on various factors, like for instance temperature, laser frequency variation or polarization state, it is necessary in a commercial system to provide an automatic control.
With Mach-Zehnder modulators (MZM), operating on a similar interferometric principle, a phase control can be realized relatively straightforward, because variations of the interference properties have a direct effect on the signal form. A DLI however cannot be controlled in the same way, because the signal, in contrast to a MZM, interferes with a delayed version of itself, resulting in no direct effect on the amplitude of the signal.
In experiments known from literature DLIs are usually manually finetuned. In a commercial system however a manual adjustment cannot be realized.
One prior art solution proposes a method for an automatic control of a DLI where the carrier must be present in the transmitted spectrum. This however disadvantageously requires a deliberate detuning of the phase modulator to a certain extent and results in a signal distortion.